Not sure if this is intended but since I am using this mod wired things are happend to may account and currently I can not login into it (MJ Support already contacted).

What I not understand is, why a random UUID is generated in Secure.java instead of requesting the players UUID buy requesting it from the MojangAPI because this would help to use this mod correctly in MP while the server is in offline mode.

This could also end up in unwanted behavior while then mojangs auth services thinks a hacker wants to hack my account because Username and UUID don't maching compared to a request over the API from Mojang.

I hope that this isn't causing all the trouble which I have but I'll see what the response from mojang will be.

At the end it would be nice to request the UUID from a given Username over the API from mojang instead of generating it randomly (only, if the request from the API does not results in a valid UUID value, then generate a random one).

Would be nice, thanks :)

Are you refering to this line? That uuid represents a server and it normally gets generated by the server at each startup - giving it the players uuid would be pointless yet working.
What i dont understand is what you mean by "this would help to use this mod correctly in MP while the server is in offline mode."